1. Field of the Invention
The invention relates to a non-volatile semiconductor memory device and a method of rewriting data stored in a non-volatile semiconductor memory device.
2. Description of the Related Art
FIG. 1 illustrates a structure of a conventional non-volatile memory cell having a floating gate electrode.
The illustrated non-volatile memory cell is comprised of a p-type semiconductor substrate 1, a first gate insulating film 4 formed on the p-type semiconductor substrate 1, a floating gate electrode 5 formed on the first gate insulating film 4 and composed of first polysilicon, a second gate insulating film 6 formed on the floating gate electrode 5 and having a three-layered structure of ONO (Oxide-Nitride-Oxide), and a control gate electrode 7 formed on the second gate insulating film 6 and composed of second polysilicon.
A source 2 and a drain 3 both comprised of a n+ diffusion layer is formed at a surface of the p-type semiconductor substrate 1 at opposite sides of the duplex gates 5 and 7.
An electric power supply 8 applies a voltage across the control gate electrode 7 and the p-type semiconductor substrate 1.
For instance, Japanese Unexamined Patent Publications Nos. 7-73688, 7-312093 and 7-326196, and Japanese Patent Publication No. 2645122 (Japanese Unexamined Patent Publication No. 2-193398) have suggested a method of deleting data stored in such a non-volatile memory cell as illustrated in FIG. 1. In accordance with the suggested method, a gradually increasing voltage is applied across the control gate electrode 7 and the p-type semiconductor substrate 1.
Hereinbelow is explained the method with reference to FIGS. 2 and 3.
In the above-mentioned method of deleting data stored in a non-volatile memory cell, a voltage which is negative relative to the p-type semiconductor substrate 1 is applied to the control gate electrode 7 to thereby discharge electrons accumulated in the floating gate electrode 5, into the p-type semiconductor substrate 1. Such discharge of electrons is called Fowler-Nordheim tunnel discharge (hereinafter, referred to simply as xe2x80x9cFN tunnel dischargexe2x80x9d).
In the method, pulses are applied at a voltage V1 to the control gate electrode 7 a predetermined number of times, when data starts being deleted, as illustrated in FIG. 2.
If data cannot be deleted even by applying the pulses at a voltage V1 to the control gate electrode 7, pulses are applied at a voltage V2 to the control gate electrode 7, as illustrated in FIG. 2. Herein, a voltage V2 is higher than a voltage V1 by a predetermined increment xcex94V1 (V2=V1+xcex94V1).
If data cannot be deleted even by applying the pulses at a voltage V2 to the control gate electrode 7, pulses are applied at a voltage V3 to the control gate electrode 7, as illustrated in FIG. 2. Herein, a voltage V3 is higher than a voltage VS by a predetermined increment xcex94V1 (V3=V2+xcex94V1).
Thereafter, a voltage applied to the control gate electrode 7 is gradually increased by a predetermined increment xcex94V1, until data stored in the non-volatile memory cell is completely deleted.
A voltage to be applied to the control gate electrode 7 is gradually increased in the following two manners.
Firstly, a voltage increment xcex94V1 is determined to be relatively high, and a step time xcex94t1 during which a voltage applied to the control gate electrode 7 is kept constant is determined to be relatively short, as shown with a solid line in FIG. 2.
Secondly, a voltage increment xcex94V2 is determined to be relatively small, and a step time xcex94t2 during which a voltage applied to the control gate electrode 7 is kept constant is determined to be relatively long, as shown with a broken line in FIG. 2.
When the voltage increment xcex94V1 is determined to be relatively high, and the step time xcex94t1 is determined to be relatively short, as shown with the solid line in FIG. 2, it would be possible hasten deleting data to a desired degree. However, if the voltage is increased just before data is deleted to a desired degree, data is over-deleted more than necessary, because a voltage increment is higher than the voltage increment xcex94V1, and hence, a level of deletion of data is over a desired level of deletion of data after the voltage has been increased, as shown with a solid line 9 in FIG. 3.
In contrast, when the voltage increment xcex94V2 is determined to be relatively small, and the step time xcex94t2 is determined to be relatively long, as shown with a broken line in FIG. 2, it would be possible to prevent data from being over-deleted in such a way as mentioned above. However, since the voltage increment xcex94V2 is smaller than the voltage increment xcex94V1, data is deleted at a lower rate, which means that it would take longer time to delete data to a desired degree, as shown with a broken line 10 in FIG. 3.
In view of the above-mentioned problem in the conventional non-volatile semiconductor memory device, it is an object of the present invention to provide a non-volatile semiconductor memory device which is capable of increasing a rate at which data is deleted without over-deletion of data.
It is also an object of the present invention to provide a method of deleting data stored in a non-volatile semiconductor memory device, which method is capable of doing the same.
In one aspect of the present invention, there is provided a non-volatile semiconductor memory device, including (a) a first gate insulating film formed on a channel region of a semiconductor substrate, (b) a floating gate electrode formed on the first gate insulating film, (c) a second gate insulating film formed on the floating gate electrode, (d) a control gate electrode formed on the second gate insulating film, and (e) an electric power source applying a gradually increasing voltage across the control gate electrode and the semiconductor substrate, the electric power source varying both an increment by which the voltage is increased and a period of time during which the voltage is kept applied, while data is being rewritten.
The electric power source may vary both the increment and the period of time at a predetermined time, in which case, it is preferable that the electric power source applies such a voltage that a first increment is smaller than a second increment and that a first period of time is longer than a second period of time. The first increment is defined as an increment by which the voltage is increased after the predetermined time, the second increment is defined as an increment by which the voltage is increased prior to the predetermined time, the first period of time is defined as a period of time during which the voltage is kept constant after the predetermined time, and the second period of time is defined as a period of time during which the voltage is kept constant prior to the predetermined time.
The electric power source may vary both the increment and the period of time at each of a plurality of predetermined times, in which case, it is preferable that the electric power source applies such a voltage that a first increment is smaller than a second increment and that a first period of time is longer than a second period of time, the first increment being defined as an increment by which the voltage is increased after the each of a plurality of predetermined times, the second increment being defined as an increment by which the voltage is increased prior to the each of a plurality of predetermined times, the first period of time being defined as a period of time during which the voltage is kept constant after the each of a plurality of predetermined times, and the second period of time being defined as a period of time during which the voltage is kept constant prior to the each of a plurality of predetermined times.
In another aspect of the present invention, there is provided a method of rewriting data stored in a non-volatile semiconductor memory device comprised of (a) a first gate insulating film formed on a channel region of a semiconductor substrate, (b) a floating gate electrode formed on the first gate insulating film, (c) a second gate insulating film formed on the floating gate electrode, and (d) a control gate electrode formed on the second gate insulating film, the method including the step of applying a gradually increasing voltage across the control gate electrode and the semiconductor substrate, wherein both an increment by which the voltage is increased and a period of time during which the voltage is kept constant are varied, while data is being rewritten.
The method may further include the steps of (a) predetermining a time, and (b) varying both the increment and the period of time at the thus predetermined time, in which case, it is preferable that the increment and the period of time are varied such that a first increment is smaller than a second increment and that a first period of time is longer than a second period of time, the first increment being defined as an increment by which the voltage is increased after the predetermined time, the second increment being defined as an increment by which the voltage is increased prior to the predetermined time, the first period of time being defined as a period of time during which the voltage is kept constant after the predetermined time, and the second period of time being defined as a period of time during which the voltage is kept constant prior to the predetermined time.
The method may further include the steps of (a) predetermining a plurality of times, and (b) varying both the increment and the period of time at each of the thus predetermined times, in which case, it is preferable that the increment and the period of time are varied such that a first increment is smaller than a second increment and that a first period of time is longer than a second period of time, the first increment being defined as an increment by which the voltage is increased after each of the predetermined times, the second increment being defined as an increment by which the voltage is increased prior to each of the predetermined times, the first period of time being defined as a period of time during which the voltage is kept constant after each of the predetermined times, and the second period of time being defined as a period of time during which the voltage is kept constant prior to each of the predetermined times.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In accordance with the present invention, a voltage increment is set relatively high and a period of time during which an increased voltage is kept constant is set relatively short, before a threshold voltage of the non-volatile semiconductor memory device reaches a predetermined final threshold voltage, whereas a voltage increment is set relatively small and a period of time during which an increased voltage is kept constant is set relatively long, after a threshold voltage of the non-volatile semiconductor memory device has reached a predetermined final threshold voltage.
Before a threshold voltage of the non-volatile semiconductor memory device reaches a predetermined final threshold voltage, a mass of electrons are accumulated in the floating gate electrode. Hence, a relatively low voltage is applied across the control gate electrode and the semiconductor substrate so as to weaken an electric field to be applied to the first gate insulating film, before a threshold voltage of the non-volatile semiconductor memory device reaches a predetermined final threshold voltage. As electrons accumulated in the floating gate electrode are discharged, that is, as data is deleted, a voltage applied across the control gate electrode and the semiconductor substrate is increased. In accordance with development of data deletion, a voltage increment is determined to be smaller and a period of time during which a voltage is kept constant is determined to be longer.
By varying a voltage to be applied across the control gate electrode and the semiconductor substrate, in the above-mentioned manner, it would be possible to weaken as much as possible an electric field applied to the first gate insulating film while data is being deleted, and to increase a rate at which data is deleted, that is, to shorten a period of time necessary for deleting data stored in a nonvolatile semiconductor memory device.
It would be also possible to prevent that a threshold voltage of the nonvolatile semiconductor memory device exceeds a predetermined final threshold voltage just before data deletion finishes, that is, data is over-deleted.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.